1. Field of the Invention
The present invention relates to a magnetic recording medium used for a hard disk device using a magnetic recording technology and to a magnetic recording/reproducing apparatus using the same.
2. Description of the Related Art
A problem of increase in noise generated from a medium has arisen as recording density becomes higher in response to a demand for a larger capacity of a hard disk device. The noise from a medium is thought to be ascribable mainly to zigzag magnetic domain walls in bit boundaries. This is because various factors such as a writing magnetic field of a head and the size of grains constituting an internal part of the medium are involved in determining the shapes of bits, and especially due to variation in grain size, positions where the bit boundaries are formed are not fixed. For noise reduction, irregularities of the recording bit boundaries have to be minimized. One way of making the irregularities of the recording bit boundaries small is to make magnetic grains forming a magnetic layer microscopic.
However, when the magnetic grains become more microscopic on one hand, a problem of deterioration in thermal fluctuation resistance occurs on the other. Here, realizing uniform grain size distribution of the grains is an effective way for making the irregularities of the boundaries of the recording bits smaller without deteriorating the thermal fluctuation resistance of the magnetic grains. However, it is thought to be difficult for a currently available film forming technology to make the grain size distribution uniform while keeping the crystal grain size at a current level (about 10 nm) or smaller.
As an art for obtaining a microscopic and uniform film, an art of forming microscopic and uniform pores made of SiO2 is disclosed in a catalyst field (see, for example, Patent document 1 below).
In this prior art, by annealing in an oxygen atmosphere after film deposition, a microscopic film having Fe2O3 grains and SiO2 grain boundaries is formed and this film is acid-etched, so that a film having the microscopic pores of SiO2 can be formed. However, this prior art is an art relating to a ceramic film having one-dimensional through pores and requires heat treatment and so on in an atmosphere which takes several to several ten hours, and thus cannot be easily applied to manufacture of magnetic recording media where a great importance is put on mass production and a film is formed mainly in a vacuum.
In another disclosed magnetic recording medium, a base film of the magnetic recording medium has a honeycomb structure composed of: crystal grains of cobalt oxide, ferric oxide, chromium oxide, and nickel oxide; and crystal grain boundaries of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide, and a control layer made of magnesium oxide, a chromium alloy, and a nickel alloy (see, for example, Patent documents 2 and 3 below).
The above prior art discloses an art relating to a magnetic recording medium that utilizes an electron cyclotron resonance (ECR) sputtering method to form the base film with the honeycomb crystal grain structure, and this base film is more uniform than that formed by a typical sputtering method. Further, it is also disclosed that the magnetic recording medium has, between the underlayer and a magnetic layer, a lattice constant control layer for adjusting a mismatch between crystal lattices of these layers, thereby inhibiting reduction in magnetic coercive force and change in magnetic characteristic caused by the mismatch between the crystal lattices. It is stated that a recording density exceeding 40 Gbits/inch2 is attained.
Here, in a metal oxide, there generally exist oxides that are stable under room temperature and atmospheric pressure and are different in oxidation number. For example, in a cobalt oxide, CoO, Co3O4, and so forth exist, and in a ferric oxide, FeO, Fe2O3, Fe3O4, and so forth exist. Neither of the above prior arts particularly discloses information on the oxidation number (valency of metal).
In order to achieve a still higher recording density exceeding 100 Gbits/inch2, a magnetic film having more uniform and microscopic crystal grains has to be formed. For this, it is desired that still more uniform and microscopic crystal grains should be formed as a underlayer. However, neither of the above prior arts can be said to have fully realized the formation of uniform and microscopic crystal grains to make magnetic grains of a recording layer more microscopic and uniform, that is, they cannot be said to have fully achieved still higher recording density exceeding 100 Gbits/inch2.
Further, Patent document 4 below discloses an invention relating to a perpendicular magnetic recording medium that has, between a soft magnetic underlayer and a magnetic recording layer, a film magnetically isolated in an in-plane direction and made of semihard ferrite or a soft magnetic granular film. This invention is characterized in that, as the semihard ferrite, γ-Fe2O3 or Fe3O4 is used, and the soft magnetic granular film is composed of soft magnetic grains and a nonmagnetic base, the soft magnetic grains being made of at least one kind of material selected from Co, Fe, and Ni, and the nonmagnetic base being made of a material selected from SiO2, Al2O3, C, and ZrO2.
In the above prior art, microscopic grains with a semihard or soft magnetic property are intended for collecting a magnetic field steeply from a head, and no implication is given that the formation of fully uniform and microscopic crystal grains of this underlayer is intended for achieving still higher density recording.                [Patent Document 1] Japanese Patent Laid-open Application No. Hei 10-182263        [Patent Document 2] Japanese Patent Laid-open Application No. 2001-134930        [Patent Document 3] Japanese Patent Laid-open Application No. 2002-163819        [Patent Document 1] Japanese Patent Laid-open Application No. 2003-228809        